Ensoniq SQ-2 (32 voice) needs to warm up before works?

Synth Repair
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1

I’ve got an SQ-2 (32 voice) that’s not working correctly, and I’m looking for help in debugging. On power on, it displays (consistent) weird characters/pixels on the LCD, button presses don’t work, and the keyboard doesn’t cause notes to play. Incoming MIDI information works to play notes however. After some time (15-60 minutes), after turning synth off and on again, the LCD starts to display more and more things normally, and then the buttons start to work, and finally the keyboard works to trigger notes.

All the power measurements (per the Service Manual) are correct. I’ve removed and re-seated all socketed chips. I’ve re-flowed all the solder joints for the DUART, the display and keyboard/panel scanner chip (6500), the processor on the display board, and the connector pins between the main board and the display/button board. I’ve sprayed DeOxit on all connectors.

So generally, it seems like this must be a heat-related issue? I’ve tried spraying compressed cold air on some of the chips (e.g. DUART and 6500), to see if cooling them down (once they were heated up and working normally) would cause them to malfunction again - no change.

Suggestions for what to try next?

2

I’d check the power supply for excessive ripple with an oscilloscope. If the ripple is over 10% (ie 50mV out of 5V) then probably time to recap the power supply. Can’t guarantee that’s the issue but it’s worth getting that sorted out for the longevity of the instrument anyway.

I’d also check that the reset line is functioning properly on the display board. I don’t see a schematic online so you’ll have to look up the datasheet for the processor. Sort of sounds like your main board is turning on just fine but the display board processor is powering up too slow and missing its chance.

3

The electrolytic caps have timed out and the unit is failing. Stop using it because if it gets worse it can damage some irreplaceable IC chips.

The power supply capacitors will need replacing, plus if any others are on separate boards, they may need replacing too. The electrolyte fluid is corrosive and eats the inside of the capacitors, and after 30 years they go out of spec.

If there are tantalum caps they fail too. Sometimes they blow up and sometimes they short without any visual indication.

Once that is done you can then trouble shoot the other issues.

4

Thanks to you both for the replies, and apologies for the tardy response, for some reason I didn’t get notification of your posts.

Have ordered the caps and plan to replace, hopefully that will sort the issue. It does seem that there’s a good amount of ripple on the 5V rail.